Patients suffering from diabetes often have to inject themselves with frequent doses of insulin and this can be done using a conventional syringe. However, the patients often also suffer from side effects of their illness and are not capable of accurately controlling the operation of such a syringe. It is therefore desirable that they should be provided with means for automatically administering an accurately controlled dosage. However, the dosage required by different patients can vary over quite wide ranges, from for example 2 units of insulin per dose to 30 or more units, and it is necessary to ensure that any device is capable of selecting a range of dosages simply and accurately.
Many forms of dispensing device use a pawl and ratchet mechanism to connect a push button or trigger operated by the user to a plunger driving a piston in the barrel of the syringe or a cartridge carried by the device. This achieves a positive drive on the forward stroke, but allows the button or trigger to be retracted, for example under the bias of a return spring, with the pawl riding over the teeth of the ratchet, in readiness for the next actuation of the device. The drive between the pawl and the ratchet is thus never fully disengaged. Typical of such devices are those described for example in U.S. Pat. Nos. 1997129, 2605763, 2718299, 3517668, 3894663, 3977574, 4022207, 4099549, 4415101, 4457712 and 4470317; French Patent Specifications Nos. 1445659, 1170312 and 1149735; and German Patent Specification No. 730971.
Where any provision is made for selecting the volume of fluid to be dispensed, this is usually by way of stops limiting the depression of the push button or trigger operating the device.
European Patent No. 0037696 describes a device in which positive drive between the plunger and the push button is achieved by having ratchet teeth along the length of the plunger into which engage the co-operating teeth of a spring loaded pawl member carried on an axially operated push member extending through the rear end of the device. A stop engaging in a slot in the push member limits the extent of travel of the push member and the volume of fluid to be dispensed is selected by withdrawing the push member the required distance from the forward extreme of its travel with the pawl riding over the teeth of the ratchet. The dose is administered by depressing the push member which carries the plunger with it. Once the plunger has reached the forward extreme of its travel and the container has been emptied, the pawl automatically disengages from the plunger to allow the plunger to be fully retracted to permit a new container to be fitted to the device.
In the above forms of device, an essential feature of the design is that the pawl is free to ride over the teeth of a ratchet as the pawl is retracted and the drive is thus not fully disconnected from the ratchet so as to be ready for driving the ratchet forward in the next delivery stroke of the device. Firstly, this does not permit a user to correct any error in setting the extent of retraction where this is used to set the amount of fluid to be dispensed. As a result, if too large a retraction has been permitted, the whole of the incorrect dose must be discharged before the device can be correctly set. Secondly, by automatically retracting the pawl in readiness for the next dose, the device is put into a "cocked" condition, which means that a user can operate the device accidentally. Thirdly, we have found that where the user is weak he may not depress the push button or trigger completely or smoothly. This may allow the pawl to retract partially or completely before it has reached the full extent of its forward travel. It will therefore appear to the user that the full dose has not been administered and he will then continue to depress the push button or trigger for its full travel. As a result, the user may administer an overdose, which could be fatal.
GB Specification No. 21096904 A describes a dispensing mechanism in which the plunger has an external screw thread and fits within an internally screw threaded fixed sleeve. The plunger is rotated by a drive cap so as to move the plunger axially. The cap incorporates a pawl and ratchet mechanism so that the cap can be rotated in one direction without rotating the plunger, but rotates the plunger in the opposite direction. The volume of fluid to be dispensed in set by rotating the cap in the first direction the desired amount from a zero point. The dose is dispensed by rotating the cap in the opposite direction back to the zero. Whilst this device is not automatically returned to the "cocked" position after each use, it is cumbersome to use, especially when the user is injecting fluid single handedly into his posterior. Furthermore, since the drive between the cap and the plunger is not fully disengaged, the device can be pumped by repeated rotation and contra-rotation of the cap. It has been proposed in PCT Published Application No. WO 85/02546 to operate a syringe using an electric stepper motor to advance the plunger in the syringe a predetermined amount. This may reduce the risk that an incorrect or excessive dose is dispensed, but such a device is expensive and cumbersome and is not suited for carriage upon the person or for general use.
It has further been proposed, for example in Swiss Patent No. 293302 and U.S. Pat. No. 2695023, to use an automatically engaging latch to limit the travel of the plunger of a syringe to the distance between adjacent notches on the plunger into which the latch engages. This permits the user to dispense only single doses. Where multiple doses are required, the user must repeatedly actuate the latch and must count and remember the number of times he has actuated the latch. This is awkward and often a user cannot remember correctly the number of times he has operated the latch, leading to inaccurate doses.
A further problem with the above devices is that a user cannot determine accurately how much insulin or other medicament is left in the body of the syringe or cartridge and hence whether he can achieve the next dosage completely from that syringe or cartridge or whether he must use a fresh one to achieve the complete dose. Mere visual inspection through the transparent wall of the container is usually too inaccurate to be able to distinguish between, say, 8 and 14 units of insulin remaining in the container and some more accurate guide is required.
As a result, a need still exists for a simple measured dose dispensing device which can deliver accurately controlled but variable doses of fluid and which can be used single handedly by weak or infirm users without the risk of "pumping" the device to administer an overdose.